Objectives We studied the effects of different soil mulching methods and biochar addition on soil aggregate stability and humus properties under no-tillage conditions in Weibei dryland. We aim to provide a theoretical basis for selecting suitable farming management measures.

Methods The field experiment lasted 18 consecutive years; we collected soil samples at 0−10 cm and 10−20 cm depth from the five no-tillage treatments, namely, no mulching control (NT), biochar application (NB), straw mulching (NS), plastic film mulching (NP), and straw plastic film binary mulching (NSP). Dry-sieving and wet-sieving methods were used to divide the soil aggregate into four fractions: silty-clay (<0.053 mm), microaggregate (0.053−0.25mm), macroaggregate (0.25−2 mm), and large macroaggregate (>2 mm), and the contents of organic carbon and humus components in the aggregates of each fraction were determined.

Results 1) Compared with NT, NS had the highest humin acid content (0.93%−92.6%) and fulvic acid content (1.8%−327.5%) across the four aggregate sizes; NSP increased the organic carbon by 1.6%−30.5%; organic carbon increased by 39.9%−161.8% in soil aggregate of NB treatment NB recorded a higher increase in humin content in 0−10 cm soil layer (87.2%−271.7%). NB, NS, and NSP improved soil aggregate stability and maize seed yield, with the most remarkable effect recorded for NSP. Compared with NT, NSP increased the mean weight diameter (MWD) of soil aggregate in 0−10 cm and 10−20 cm soil layers by 48.6% and 73.5%, the geometric mean diameter (GMD) in 0−10 cm and 10−20 cm soil layers by 59.2% and 63.1%, and the corn grain yield by 25.8% in 2021. 2) Humin was dominant in the organic carbon, accounting for 37.6%−91.3% of the total organic carbon in the water-stable aggregates in different particle sizes. NB (P<0.05) increased the ratio of humic to fulvic acid (HA/FA) in most water-stable aggregates, with the maximum increase (80.2%) recorded in micro-aggregates in 0−10 cm soil layer. Plastic film mulching (NP and NSP) decreased the HA/FA of humus (6.8%−27.6%), and the humic acid E4/E6 in silty clay (P<0.05 ) decreased (4.2%−6.6%).

Conclusions Under no-tillage, both straw mulching and biochar application (NS, NSP, and NB) can improve aggregate organic carbon content, stability, and corn yield, but their effect on the humification degree of organic carbon is different. Straw mulching (NS) is the most effective for humic and fulvic acid content in humus. Biochar application is the most effective in increasing the humin content and its proportion in organic carbon. Straw and plastic film binary mulching (NSP) improves aggregate stability, organic carbon content, and corn grain yield, proving the best treatment under the test condition and area.